Abstract

The sulfur isotopic compositions of sedimentary sulfates
and sulfides are useful for understanding modern microbial
ecology and for reconstructing paleoenvironmental conditions
associated with the deposition of ancient sediments. In many
modern microbially-dominated sedimentary environments,
such as microbial mats, or methane seeps, the redox gradients
can be steep with the transition from oxic to sulfidic condition
over the space of mm to cm. In these environments, it is
frequently difficult (either for logistical reasons or sample
volume requirements) to sample at a sufficiently high
resolution to capture the geochemical and microbiological
details associated with these redox transitions. We build upon
earlier work [1] to demonstrate the ability to capture aqueous
sulfide as silver sulfide, which can then be analyzed using a
Cameca NanoSIMS 50L or 7F/Geo for its isotopic
composition at a spatial resolution down to ~ 1 - 50 um. This
allows for the construction of 2D isotopic datasets that
document vertical isotope gradients as well as lateral
heterogeneity [2]. Here we present the application of this
sulfide capture technique to three different modern
environments: (1) microbial mats from Guerrero Negro, Baja
California Sur, Mexico; (2) the chemocline of meromictic
Lake Mahoney, British Columbia, Canada; and (3) methane
seep-associated marine sediments offshore Costa Rica.
Coherent variations up to 20 permil in δ34S are observed over
ranges as small as 1 mm at all depths examined. These data
highlight the additional ecological information that can be
extracted from high resolution isotopic data, which may
improve our understanding of the activity of the microbial
ecosystems driving biogeochemical cycling in these systems.